1.Field of the Invention
The present invention is directed to a surface coil arrangement of type suitable for transmitting and receiving radio-frequency signals in a nuclear magnetic resonance apparatus.
2.Description of the Prior Art
A surface coil arrangement for conducting an arrangement of a subject in a nuclear magnetic resonance tomography apparatus is described in the article "Rotator Cuff Tears, Preliminary Application of High-Resolution MR Imaging with Counter Rotating Current Loop-Gap Resonators," Kneeland et al., Radiology, 1986, pages 695-699. This surface coil arrangement consists of two identically constructed surface coils arranged geometrically anti-parallel relative to each other, and being electrically connected to each other so that high-frequency magnetic fields which uniformly penetrate both surface coils do not generate a resultant current.
In the use of surface coils for receiving nuclear magnetic resonance signals and whole-body antennas for exciting nuclear magnetic resonance signals in an examination subject, it is a constant problem to decouple the surface coil from the radio-frequency transmitting field. If this is not done, the current induced in the surface coil would generate a radio-frequency field which would be superimposed on the excitation field. This would result in disturbances in the nuclear magnetic resonance imaging, as well as in nuclear magnetic resonance spectroscopy. Excessive local concentrations of the radio-frequency field acting on the patient could also be generated undesireably due to a surface coil in resonance. Moreover, damage to the reception system could be caused by over-voltages arising in the surface coil.
This problem can be avoided in linearly polarized radio-frequency fields by aligning the surface coil parallel to the excitation field. This solution, however, causes a number of operational limitations. Moreover this solution has no effect at all in circularly polarized excitation fields.
Two loop-gap resonators are described in the aforementioned article, which have identical areas and which are arranged geometrically anti-parallel to each other in an effort to solve this problem. The two resonators are electrically cross-connected at the gaps.
This coil arrangement is insensitive to irradiation by a uniform excitation field, but is more sensitive to the radio-frequency emission emanating from the examination subject in the reception mode. This can be explained by the following radio-frequency oriented considerations.
The electrical wiring of the two surface coils is geometrically anti-parallel, i.e., the respective loops forming the coils are wound in opposite directions. This results in two resonant circuits having the same resonant frequency. Due to the close geometric proximity of the coils, as well as the aforementioned circuit connection, a system of two highly coupled resonant circuits is achieved, which exhibits two separate resonant frequencies under the respective conditions of the current flowing the same direction in the coils, or the current flowing in opposite directions in the coils. Adjustment can be made so that the resonant frequency resulting from oppositely flowing currents is equal to the operating frequency of the nuclear magnetic resonance apparatus. The coil arrangement is thus sensitive for the reception of nuclear magnetic resonance radio-frequency signals from a non-uniform radio-frequency field distribution emanating from the examination subject, however, is not sensitive for the uniform excitation field which would excite isodirectional currents whose resonance frequency is sufficiently different from the operating frequency. The surface coil arrangement constructed in this manner is thus decoupled from the uniform excitation field independently of the position of the surface coil arrangement in relation to the field direction. This is also true for circularly polarized excitation fields.
The balancing as well as the line matching of surface coil arrangements is generally greatly dependent on the load represented by the examination subject. A re-balancing for each different examination subject is therefore generally required. This results in a lengthening of the examination times, and also introduces an uncertainty or fluctuation in the examination results, mainly caused by movements of the examination subject during the examination.
This surface coil arrangement is also sensitive to irradiation by electrical noise emission from external sources.
A surface coil which is only slightly detunable, and is thus relatively insensitive to noise radiation, is described in European application No. 0 222 982, corresponding to U.S. Pat. No. 4,816,766. This surface coil is formed by a coaxial line section shaped into a loop, and having jacket shielding with a gap or interruption at one location within the loop. Both the inside conductor and the jacket shielding at one end of the coaxial line section are electrically connected to the jacket shielding of the other end of the line section in the loop. Due to the action of the jacket shielding, this surface coil is insensitive to capacitive detuning due to external influences, as well as to external, electrical noise emissions. The problem of decoupling from the excitation field, however, remains.